The subject invention is directed to a sheet product which is useful as separators in batteries, in particular, separators in lithium batteries to prevent the occurrence of overheating and thermal-runaway.
Storage batteries have at least one pair of electrodes of opposite polarity and, generally, have a series of adjacent electrodes of alternating polarity. The current flow between these electrodes is maintained by an electrolyte which can be acid, alkaline or substantially neutral depending on the nature of the battery system. Separators are located in the batteries between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates while freely permitting electrolytic conduction. The separator is normally in the form of a thin sheet or film or in certain designs can be in the form of an envelope surrounding each electrode plate of one polarity. It is generally agreed that separators should be (a) thin and light weight to aid in providing a battery of high energy density, (b) resistant to degradation and instability with respect to the battery components with which it is in contact, (c) capable of exhibiting a high degree of electrolytic conductivity (low electrolytic resistance) and (d), in battery systems where appropriate, capable of inhibiting formation and growth of dendrites.
Lithium batteries have distinct advantages over other storage batteries. These batteries are capable of providing much higher power storage densities than other types of batteries, excellent shelf life, high energy density (power capability per unit weight) due to the low atomic weight of lithium metal, and high potential for forming a battery in conjunction with positive electrodes far removed from the lithium electrode in the electromotive series. The battery can be formed in any conventional physical design--cylindrical, rectangular or disc-shaped "button" cells--normally of a closed cell configuration. Such batteries are generally composed of a negative lithium electrode, a positive electrode and a non aqueous electrolyte. The negative electrode is conventionally lithium metal or its alloy on a support, such as a nickel coated screen. Positive electrodes of various types have been suggested, including metal oxides such as manganese dioxide or transition metal sulfides such as sulfides of cobalt, nickel, copper, titanium, vanadium, chromium, cerium and iron. The positive electrode may further contain carbon and a current collector. The electrolyte is formed of a non aqueous solvent containing a lithium salt. For example, solvents may be acetonitrile, tetrahydrofuran, propylene carbonate, and various sulfones. The lithium salts may be lithium perchlorate, iodide or hexafluroarsenate and the like. An additional, normally passive, component of the battery is a separator membrane located between plates of opposite polarity to prevent contact between such plates while permitting electrolytic conduction.
Separators conventionally used in present battery systems are formed of polymeric films which when placed in an electrolyte or electrolyte system, are capable of exhibiting a high degree of conductivity while being stable to the environment presented by the battery. The films may be macroporous or microporous to thus permit transportation of electrolyte. Examples of such separators include polypropylene sheet which has been stretched and annealed to provide microporosity in the sheet. Such sheets are normally highly oriented and shrink when subjected to heat. Other examples are filled polymeric sheets such as those disclosed in U.S. Pat. Nos. 3,351,495 and 4,287,276 in which the electrolyte is capable of passing through the separator through microporous channels and by the wicking of the filler.
Due to the reactivity of lithium, a major problem encountered with these batteries involves overheating of the cell due to improper use of the cell, e.g. placed in inverted position causing plating out of electrolyte onto an anode; contact between electrodes of opposite polarity such as by dendrite formation or shrinkage of separator; formation of high surface spongy lithium which exothermally reacts with the solvent-electrolyte; as well as other known conditions. Such overheating tends to cause thermal runaway and potentially explosive effects as the system continues to act in its defective mode. This must be controlled to provide a battery having commercial acceptability.
Polymeric films presently employed as separators in lithium batteries are generally not capable of preventing any uncontrolled overheating. Some separator films are inert to heat and, therefore, do not trigger any preventative mechanism. Other presently used separators, such as microporous polyolefins, when subjected to elevated temperatures exhibit dimensional instability and/or degradation permitting contact between electrodes of opposite polarity. Such instability only accelerates the thermal-runaway of a battery.